Dell made an energy-friendly announcement today, with the introduction of their energy-efficient Inspiron 531. The new desktop is Energy Star 4.0 rated, coupling together a highly efficient power supply, low power components and even a paperless manual (although that might irritate some). All in all, Dell claims that using this machine in place of another represents significant carbon output reduction. Interestingly, they equated it to cars:

Using an energy-efficient desktop with a flat panel monitor for four years can reduce carbon dioxide (C02) emissions by approximately the same amount as not driving a car for eight weeks.
The desktop is extremely cheap at $369, and of course is aimed at low-end desktop users rather than anything else. However, it still sports a dual-core processor, though the obvious selling point here is the continued reduction in power. This is yet another mark of a changing market in which raw performance is becoming less of a concern, and efficiency is playing a much larger role. You can read the full press release in the comments section.